Pope Francis said Republican presidential candidate Donald Trump is "not Christian" because of his views on immigration Feb. 18. Pope Francis was speaking to reporters on his way back to Rome from Mexico. (Reuters)

Pope Francis hasn't been afraid to wade into the depths of American politics. Depending on your political views, he's made controversial statements on everything from climate change to abortion to immigration.

Now, it appears the pope has willingly jumped into a war of words with none other than GOP presidential front-runner Donald Trump.

Aboard the papal plane on his way back from Mexico, reporters asked the pope to respond to Trump's signature campaign proposal, to build a wall along the border with the U.S. and Mexico. The pope's response:

“A person who thinks only about building walls — wherever they may be — and not building bridges, is not Christian."

Trump fired back the pope's comment was "unbelievable" and "disgraceful."

The whole affair really is kind of mind-blowing. Trump, whose brand is brash confrontation, has finally picked arguably the biggest fight of all — with a man millions believe speaks directly to God himself. It was a bold move for both men.

But the Trump-Francis feud isn't the first time a pope has placed himself directly in front of the train that is another country's politics. It's actually happened quite a few times in modern papal history. Precedents suggest that some of the popes have stepped in when they think a country's leaders have strayed too far from democracy or Christian ideals.

Here's a list of some of the top papal political feuds to date, in chronological order:

Pope John Paul II was among the Catholic church's most active leaders in global politics, traveling to more than 120 countries during his nearly three-decade tenure. One of his first international headlines on politics came on his first official trip to the Philippines. There, Pope John Paul II was a popular man, welcomed by massive crowds nearly everywhere he went.

The Philippines' president/dictator, Marcos, was well aware of the pope's popularity — and the pope's criticism of human rights abuses under his government. Marcos lifted martial law a month before the pope's visit, though he kept much of his dictatorial powers.

In a speech with the president sitting nearby, the pope explicitly criticized the Marcos regime, saying, "Even in exceptional situations that may at times arise, one can never justify any violation of the fundamental dignity of the human person or of the basic rights that safeguard this dignity."

It was Marcos's turn to speak next; he apparently apologized to the pope for "petty and small" conflicts between the church and state.

In the 1980s, a leading Nicaraguan priest became involved in the country's political revolution, eventually helping lead the left-wing Sandinistas. At the time, church involvement in government positions was a no-no. Pope John Paul II publicly reprimanded Miguel D'Escoto Brockmann, and the Vatican suspended him and two other priests for their involvement.

In 2015, an 81-year-old D'Escoto asked the Vatican to reinstate him so that he could celebrate Mass again "before dying." Under Pope Francis, he was welcomed back into the church.

3. Pope John Paul II vs. Chilean President Augusto Pinochet, 1987

While speaking to reporters aboard a plane (sensing a theme here?) en route a two-week South American tour, John Paul II said Chile's Pinochet-ruled government was "dictatorial."

Pinochet, who ran Chile from the 1970s to 1990 and presided over thousands of politically linked deaths, imprisonments and disappearances, had recently said of the country's bishops who were critical of his government's human-rights abuses: "It would be better if they spent 90 percent of their time praying.''

Asked if he expected to help bring democracy to Chile, the pope said: ''Yes, yes, I am not the evangelizer of democracy, I am the evangelizer of the Gospel. To the Gospel message, of course, belongs all the problems of human rights, and if democracy means human rights it also belongs to the message of the church.''

4. Pope John Paul II vs. Bill Clinton, 1994

Pope John Paul II and Clinton clashed publicly several times on the issue of abortion.

While visiting the U.S. in 1993 via an international Catholic youth festival, the pope and Clinton met for the first time. The two said they had a warm private meeting, but then in public remarks, with Clinton standing by his side, the pope criticized the president for his pro-abortion-rights stance.

"If you want equal justice for all and true freedom and lasting peace, then, America, defend life," he said. "All the great causes that are yours today will have meaning only to the extent that you guarantee the right to life and protect the human person."

The Los Angeles Times reported at the time the pope's comments were non-confrontational. But a year later, the two had another open disagreement at a United Nations summit in Cairo about abortion and the availability of birth control.

5. Pope Benedict XVI vs. Turkey, 2000s

A year before he became pope in 2005, Benedict told French daily newspaper Le Figaro that Turkey shouldn't be allowed into the European Union in part because of the number of Muslims in the country.

In 2006, journalists accompanying Benedict on a trip to Turkey reported that he apparently tried to walk back those comments. But in an official declaration submitted later, the pope said Turkey's admittance into the E.U. must be dependent on religious freedom.

"In every step towards unification, minorities must be protected, with their cultural traditions and the distinguishing features of their religion," he said.

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